Direct conversion rf transceiver for wireless communications

ABSTRACT

A single chip radio transceiver includes circuitry that enables received wideband RF signals to be down converted to base band frequencies and base band signals to be up converted to wideband RF signals prior to transmission without requiring conversion to an intermediate frequency. The circuitry includes a low noise amplifier, automatic frequency control circuitry for aligning the LO frequency with the frequency of the received RF signals, signal power measuring circuitry for measuring the signal to signal and power ratio and for adjusting frontal and rear amplification stages accordingly, and finally, filtering circuitry to filter high and low frequency interfering signals including DC offset.

CROSS-REFERENCE To RELATED APPLICATIONS

The present U.S. Utility Patent Application claims priority pursuant to35 U.S.C. § 120, as a continuation, to the following U.S. Utility PatentApplication which is hereby incorporated herein by reference in itsentirety and made part of the present U.S. Utility Patent Applicationfor all purposes:

-   -   1. U.S. Utility application Ser. No. 11/743,143, entitled “A        Direct Conversion RF Transceiver for Wireless Communications,”        (Attorney Docket No. BP1985C), filed May 1, 2007, pending, which        claims priority pursuant to 35 U.S.C. § 120, as a continuation,        to the following U.S. Utility Patent Application which is hereby        incorporated herein by reference in its entirety and made part        of the present U.S. Utility Patent Application for all purposes:        -   a. U.S. Utility application Ser. No. 10/052,870, entitled “A            Direct Conversion RF Transceiver for Wireless            Communications,” (Attorney Docket No. BP1985), filed Jan.            18, 2002, now U.S. Pat. No. 7,212,586.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications and, moreparticularly, wideband wireless communication systems.

2. Related Art

Super-heterodyne receivers traditionally receive an RF signal that mustbe converted to base band by way of an intermediate frequency (IF).Thereafter, the IF signal is amplified and filtered to define acommunication channel. In a transmitter, similarly, a base band signalis up converted to the intermediate frequency wherein the amplificationand subsequent filtering are carried out at the IF stages. While somesystems skip the IF conversion step, wideband systems typically requireconversion to IF stages. Depending on the signal bandwidth and the typeof communication system, semiconductor devices are not yet able to allowfull integration of active filters operating at the elevatedintermediate frequencies for a wideband or high data rate communicationnetwork. To carry out filtering at the intermediate frequencies, surfaceacoustic wave filters (SAW) are commonly used. The SAW filters have thedrawback, however, of being bulky, heavy and expensive. Additionally,the SAW filters require low impedance matching thereby resulting in highpower consumption. Because they are often powered by battery, portablewireless communication devices are not readily adaptable for suchsystems in that they are required to be inexpensive, light and consumelower amounts of power. Thus, there is a need to design transceiversystems that eliminate the use of intermediate frequency filters.

An alternate approach to using a higher intermediate frequency thatrequires the SAW filters is to convert the RF signal to an intermediatefrequency that is sufficiently low to allow the integration of on-chipchannel selection filters. For example, some narrow band or low datarate systems, such as Bluetooth, use this low intermediate frequencydesign approach.

One problem using low intermediate frequencies, however, is satisfyingimage rejection requirements for the systems. The image rejectionrequirement for the down conversion is hard to meet and is usuallylimited to about −40 dB. Thus, this low intermediate frequency approachis limited for narrow band or low data rate systems. Wide band or highdata rate systems require an intermediate frequency that is not lowenough for the integration of channel selection filters given thetechnology that is available today for semiconductor processes. There isa need, therefore, for a wireless transceiver system that allows forfull integration on-chip of circuit designs that support high data rateand wideband communications.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods of operationthat are further described in the following Brief Description of theDrawings, the Detailed Description of the Invention, and the claims.Other features and advantages of the present invention will becomeapparent from the following detailed description of the invention madewith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredwith the following drawings, in which:

FIGS. 1A, 1B, 1C and 1D are frequency response curves and FIG. 1E is ablock diagram that illustrate some of the challenges that exist fordeveloping zero IF systems that are all integrated within asemiconductor device;

FIGS. 2A and 2B illustrate frequency response curves that are realizedby the present inventive system or transceiver;

FIG. 3 is a flowchart illustrating an overall method performed by theinventive transceiver according to one embodiment of the presentinvention;

FIG. 4 is a flowchart that illustrates a method for adjusting thechannel frequency to a desired channel frequency according to oneembodiment of the present invention;

FIG. 5 is a flowchart that illustrates a method for amplifying areceived signal in a transceiver according to one embodiment of thepresent invention;

FIG. 6 is a functional block diagram of a transceiver formed accordingto one embodiment of the present invention;

FIG. 7 is a functional schematic diagram of a transceiver formedaccording to one embodiment of the present invention; and

FIG. 8 is a functional schematic diagram of an automatic frequencycontrol (AFC) circuit formed according to one described embodiment ofthe invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Generally, the present invention provides a transceiver that allows forwideband systems formed on a chip that allow for up and down convertingfrom base band and radio frequency without conversion to an intermediatefrequency (zero IF).

FIGS. 1A, 1B, 1C and 1D are frequency response curves that illustratesome of the challenges that exist for developing zero IF systems thatare all integrated within a semiconductor device. Referring now to FIG.1A, a signal is transmitted over a wireless medium as an RF signal showngenerally at 104. For processing by a receiver, however, that signal isfirst down converted to an intermediate frequency (IF) shown generallyat 108, wherein some preliminary processing occurs. Thereafter, thesignal is down converted from intermediate frequency 108 to base bandfrequency 112.

The foregoing discussion about SAW filters may be considered in view ofthe frequency shown generally at 116. If the intermediate frequency islow enough, then the filters may be developed on chip. As describedpreviously, however, the image rejection of the on chip filters is notalways satisfactory. Thus, it is desirable to develop a zero IF system,meaning that no intermediate frequencies are used, as is illustrated inFIG. 1B, in order to satisfy image rejection requirements. Accordingly,received signals are transmitted directly from the RF signal 104 to thebase band frequency 112 as is shown in FIG. 1B. Similarly, signals thatare to be transmitted are up converted from base band frequency 112 toRF signal 104.

One problem with down converting signals directly from RF signal 104 tobase band frequency 112 is that the process of down converting thesignal immediately results in a DC offset 120, as is shown in FIG. 1C.Additionally, a noise component, often described as a 1/f interference,is illustrated in FIG. 1D. As may be seen, the 1/f interference is veryhigh at low frequencies but tapers off as the frequency is increased.One problem with the DC offset and the 1/f interference is that anyamplification of the received signal includes amplification ofinterference and/or DC power from the DC offset thereby saturating theamplifier with signals other than the received or target signal.

FIG. 1E further illustrates the process that generates most of the DCoffset. For example, a local oscillator (LO) 130 often produces leakagecurrent that is conducted into the input of an amplifier or a mixer.More specifically, as may be seen in FIG. 1E, a local oscillator 130 hasleakage current that is conducted into the input of low noise amplifier(LNA) 134 and the input of mixer 138. This type of self mixing producesthe most of DC offset at the output of the mixer 138. It is veryimportant, therefore, to eliminate these leakage currents so that the DCoffset is at a minimum level.

FIGS. 2A and 2B illustrate frequency response curves that are realizedby the present inventive system or transceiver. Referring now to FIG.2A, a DC offset is shown at 204, while the low end of a received signalfrequency is shown at 208. FIG. 2B illustrates a high pass (HP) filter220 that eliminates the DC-offset 204 and a low pass (LP) filter 214that selects the desired signal channel by attenuating higher frequencyinterference. In reality, with limited accuracy of local oscillationfrequency due to cheap reference crystal is used, if received signalcould be down converted too low that it could be attenuated by the HPfilter 220. And it could be down converted too high that is could beattenuated by the LP filter 214. In order to avoid signal degradation,automatic frequency control (AFC) is proposed as show in FIG. 2A.Accordingly, the invention includes a transceiver that determines thedifference between frequency 208 and ideal frequency 212 (as shown inFIG. 2B) and adjusts LO frequency so that the low end of the receivedsignal is located at 212 and the high end of the signal is located at216.

FIG. 2B illustrates that the down converted signal after LO frequencycorrection is located in the desired frequency range, wherein the lowend of the frequency is at 212 and the high end is at 216. As may beseen, the channel for the received signal now ranges from the frequencyshown at 212 to the frequency shown at 216. Moreover, FIG. 2B shows ahigh pass filter frequency response curve 220. As may be seen, thechannel of the received signal is well beyond the attenuation part of HPfilter curve 220. Without adjusting the frequency of LO, the high passfilter, whose frequency response curve is shown in FIG. 2B, would havefiltered or eliminated some of the received signal thereby losinginformation. Thus, FIGS. 2A and 2B suggest that the inventive systemincludes circuitry for not only correcting LO frequency, but also tofilter the received signal thereafter with a high pass filter and a lowpass filter.

FIG. 3 is a flowchart that illustrates an overall method performed bythe inventive transceiver according to one embodiment of the presentinvention. Referring now to FIG. 3, a first process step taken by thetransceiver is to amplify a received RF signal with a low noiseamplifier (step 304). Thereafter, the frequency of the received signalis adjusted by LO frequency with an automatic frequency controlcircuitry. In the described embodiment, a coarse adjustment is made(step 308), as well as a fine adjustment that is made in the digitaldomain (step 312). Thereafter, the signal is down converted from aspecified RF channel to a specified base band channel (step 316) and alow pass filter is applied to eliminate interference occurring above thechannel (step 320). Thereafter, a DC offset and low frequencyinterference (e.g., 1/f) is removed with at least one high pass filtertuned to pass the base band channel (step 324). Finally, the signals areamplified by a plurality of amplifiers. The amplification level of theamplifiers is adjusted in an inverse proportional manner according tointerference levels so that total amplification remains constant (step328).

FIG. 4 is a flowchart that illustrates a method for adjusting thechannel frequency to a desired channel frequency according to oneembodiment of the present invention. Referring now to FIG. 4, theinventive method includes initially measuring a center frequency for thereceived RF signal and determining the difference between that centerfrequency and the center frequency of a specified RF channel (step 404).Initially, a coarse difference is measured and is corrected by adjustingLO frequency. Then, the residual difference is adjusted to a fine degreeof measurement in the digital domain to obtain an accurate differencebetween an actual center frequency and a specified center frequency(step 408). The difference in center frequencies is then transmitted toa signal generator (step 412). In the described embodiment of theinvention, the signal generator for the transceiver is one that iscapable of performing quadrature phase shift keyed modulation ofsignals. Accordingly, the difference in center frequency valuesdetermined in step 404 is transmitted to a sine and a cosine element ofan encoder or signal generator.

After the difference in frequency has been sent to the sine/cosineencoders, the signals are transmitted from the encoders to adigital-to-analog converter (step 416). Thereafter, thedigital-to-analog converter transmits the signals to a low pass filterto remove high frequency interference (step 420). Thereafter, the signalis transmitted to a mixer to produce a new local oscillator signaloutput. The new local oscillator output signal is characterized by thedesired frequency channel (step 424).

FIG. 5 is a flowchart that illustrates a method for amplifying areceived signal in a transceiver according to one embodiment of thepresent invention. The method of FIG. 5 generally includes using aplurality of received signal strength indicators (RSSI) to sense thepower of the received interference and signal to determine a rightamplification of cascaded amplifier stages. Initially, a first RSSI isused to sense the power of the received interference and signal (step504). Thereafter, a second RSSI is used to sense the power of the signalwithout the interference (step 508). After measuring the power of thesignal, as well as the power of the interference and signal, thetransceiver evaluates the ratio of signal power to signal andinterference power to determine optimal amplification techniques by eachof a plurality of amplifiers (step 512). If the interference level ishigh, the gain of a first amplifier is set to a lower value and the reargain of a second amplifier, which is located after channel selectionfilter, is set to a higher value in a multi-amplifier system (step 516).If the interference value is relatively low, the frontal gain is set toa higher value and the rear gain is set to a lower value (step 520). Asthe gain of the frontal and rear amplifiers are adjusted, they areadjusted in a manner wherein the total amplification is kept at aconstant level required for certain power level of desired channel orsignal (step 524). In the described embodiment, an LNA is used for thefront end and three high pass variable gain amplifiers (HP-VGA's) areused in subsequent stages.

FIG. 6 is a functional block diagram of a transceiver formed accordingto one embodiment of the present invention. Referring now to FIG. 6, atransceiver 600 includes a transceiver port 602 for receiving andtransmitting communication signals. In the described embodiment of theinvention, transceiver port 602 receives signals transmitted at the RFand generates signals that are transmitted externally at the RF.

In addition to transceiver port 602, transceiver 600 further includes aplurality of RSSIs 606 and 608 that are for sensing the power level ofthe received signals and, more particularly, of the received signal aswell as the received signal and interference. Transceiver 600 furtherincludes a pair of low pass filters 614 and 616 and an automaticfrequency control (AFC) circuit 620. Automatic frequency control 620 isfor adjusting the LO frequency in the zero IF transceiver 600 to alignwith the desired frequency channel. In the described embodiment, AFC 620adjusts the frequency of the LO frequency so that the received signal islocated within the un-attenuated part of HP and LP filters. Transceiver600 further comprises an A-D and D-A conversion circuitry 624 that isfor converting signal formats as required. Additionally, transceiver 600includes a base band processor 628 that is for processing the receivedsignal and the signal to transmit. Transceiver 600 further includes upconversion circuitry 636 that receives signals that are to betransmitted at base band from base band processor 628 and then upconverts the base band signals to the RF for transmission fromtransceiver port 602. Finally, transceiver 600 includes down conversioncircuitry 604 for converting a received RF signal to base bandfrequencies.

In operation, transceiver port 602 receives RF signals and converts thesignals from the RF to base band. The down conversion is performed bydown conversion circuitry 604 of FIG. 6. Once the signal has been downconverted, the RSSI filters 606 and 608 sense the power of the signal,as well as the signal plus interference, to determine the manner inwhich the amplification stages should be set for the received signal.While transceiver 600 shows a pair of low pass filters 614 and 616 whichare used as a part of filtering higher frequency interference during thedown conversion process as well as during the automatic frequencycontrol or adjustment process by AFC 620, it is understood thattransceiver 600 may include more than or less than two low pass filters.In general, low pass filters 614 and 616 represent the low passfiltering that occurs during the down-conversion process as well asduring the automatic frequency control process to adjust the frequencyof the received signals. Thus, in addition to sensing the power levelsof the signal and interference of the received signal, the frequency isadjusted by AFC 620 at which time it is filtered by high pass filter toremove DC offset and the 1/f interference. After the low frequencyinterference has been removed, as well as the high frequencyinterference from the various filters, the signal is amplified andconverted into digital domain for processing by the base band processor.The signal is amplified by LNA amp 610 and HP-VGA amps 612, whose totalamplification is kept at a constant value (for a certain power level ofreceived signal) but whose individual amplification is either increasedor decreased according to the signal and signal plus interference ratiosdescribed earlier.

FIG. 7 is a functional schematic diagram of a transceiver formedaccording to one embodiment of the present invention. Referring now toFIG. 7, a transceiver system comprises radio circuitry 704 that iscoupled to base band processing circuitry 708. The radio circuitry 704performs filtering, amplification, frequency calibration (in part) andfrequency conversion (down from the RF to base band and up from baseband to the RF). Base band circuitry 708 performs the traditionaldigital signal processing in addition to partially performing theautomatic frequency control. As may be seen, the single chip radiocircuitry 704 is coupled to receive radio signals that are initiallyreceived by a transceiver and then converted by a Balun signal converterwhich performs single end to differential conversion for the receiver(and differential to single end conversion for the transmitter end). TheBalun are shown to be off chip in FIG. 7, but they may be formed on chipwith radio circuitry 704 as well.

More specifically, radio circuitry 704, and more particularly, portion704A, includes a low noise amplifier 712 that is coupled to receive theRF from a transceiver port. The low noise amplifier 712 then produces anamplified signal to a mixer 716 that is for adjusting and mixing the RFas a part of the automatic frequency control that is performed by theradio and base band circuits 704 and 708. The outputs of the mixer (Iand Q of a quadrature phase shift keyed signals) are then produced to afirst HP-VGA stage 720.

The outputs of the first HP-VGA stage 720 are then produced to a firstRSSI 728 as well as to a low pass filter stage 724. The outputs of thelow pass filter stage 724 are then produced to a second RSSI 732, aswell as to a second HP-VGA 736 and third HP-VGA 740 as may be seen inFIG. 7.

In operation, the first RSSI measures the power level of the signal andinterference. The second RSSI measures the power level of the signalonly. The base band processing circuitry 708 then determines the ratioof the RSSI measured power levels to determine the relative gain leveladjustments of the front and rear stage amplification stages. In thedescribed embodiment of the invention, if the power level of the signaland interference is approximately equal to or slightly greater than thepower level of the signal alone, then the first amplification stages areset to a high value and the second amplification stages are set to a lowvalue.

Conversely, if the power level of the signal and interference issignificantly greater that the power of the signal alone, therebyindicating significant interference levels, the first amplificationstages are lowered and the second amplification stages are increasedproportionately.

Automatic frequency control circuit 704B includes low pass filters forfiltering I and Q signals and mixer circuitry for actually adjusting LOfrequency. The operation of mixers and phase locked loop for adjustingfrequencies is known. Circuit 704B further includes JTAG (Joint TestAction Group, IEEE1149.1 boundary-scan standard) serial interface (SIO)circuitry 744 for transmitting control signals and information tocircuit portions 704A (e.g., to control amplification levels) and toportion 704B (e.g., to control or specify the desired frequency for theautomatic frequency control).

A portion of the automatic frequency control circuitry that determinesthe difference in frequency between a specified center channel frequencyand an actual center channel frequency for a received RF signal isformed within the base band circuitry in the described embodiment of theinvention. This portion of the circuitry includes circuitry thatcoarsely measures the frequency difference and then uses measures thefrequency difference in the digital domain to obtain a more precisemeasurement.

Finally, radio circuitry portion 704C includes low pass filtrationcircuitry for removing any interference that is present after base bandprocessing as well as amplification, mixer and up converter circuitryfor preparing a base band signal for transmission at the RF.

FIG. 8 is a functional schematic diagram of an automatic frequencycontrol (AFC) circuit formed according to one described embodiment ofthe invention. The AFC circuit of FIG. 8 comprises a RF signalprocessing portion 804 and a base band signal processing portion 808.Generally, portion 804 is for adjusting LO frequency. Portion 808 is fordetermining the difference in center channel frequencies between thereceived RF and the expected frequency value for the received signal.

Analog-to-digital converters (ADC) 812 are used to convert the receivedanalog signal into digital. ADC 812 is coupled to provide the receivedRF signal in a digital format to a frequency synchronization circuitry820 that measures the frequency difference in a coarse degree ofresolution. Digital frequency control circuitry 816 performs itsmeasurements and calibration in the digital domain and provides itsresults to frequency synchronization circuitry 820 to adjust thefrequency difference of frequency synchronization circuit 820 with afine degree of resolution.

Frequency synchronization circuit 820, as a part of determining thedifference in center channel frequency for the received signal and anexpected value, receives and interprets a pilot signal that defines theexpected center channel frequency. Accordingly, after measuring theactual center channel frequency of the received RF, frequencysynchronization circuit 820 is able to determine the frequencydifference. Frequency synchronization circuit 820 then produces a signaldefining the difference in center channel frequency for the receivedsignal and an expected value to signal generator 824. It is understoodthat the pilot channel is transmitted as a part of standard wirelessnetwork communication protocols for signal control and synchronizationpurposes.

Signal generator 824, upon receiving the difference in center channelfrequency for the received signal and an expected value, producesquadrature phase shift keyed (I & Q) outputs for the received frequencydifference to a pair of digital to analog converters (DAC) 828. Theanalog outputs of DAC 828 are then passed to low pass filters 832 andare then up converted back to the RF. The I and Q RF signal componentsare then produced to mixer circuitry 836 that also receives a specifiedinput from phase locked loop circuitry 840 to produce a received RFhaving a specified center channel frequency. It is understood that mixercircuitry 836 (including PLL circuitry 840) further receives controlsignals from base band processing circuitry (not shown in FIG. 8)specifying the expected center channel frequency that is specified inthe aforementioned pilot channel.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims. As may beseen, the described embodiments may be modified in many different wayswithout departing from the scope or teachings of the invention.

1. A transceiver, comprising: a transceiver port for receiving andtransmitting high data rate communication signals at radio frequency;automatic frequency control circuitry operably disposed to receivecommunication signals received at radio frequency, the automaticfrequency control circuitry for adjusting a local oscillation frequencybased upon a detected difference between an actual frequency of thereceived communication signals and an expected frequency of the receivedcommunication signals wherein the automatic frequency control circuitryproduces an adjusted local oscillation; down conversion circuitry toreceive the adjusted local oscillation from the automatic frequencycontrol circuitry and further coupled to receive the communicationsignals at radio frequency wherein the down conversion circuitry isoperable to produce base band frequency communication signals based uponthe adjusted local oscillation and upon the received communicationsignals at radio frequency; first received signal strength indicationcircuit for measuring power levels of signal and interference from anode disposed up-stream of low pass filtering circuitry; second receivedsignal strength indication circuit for measuring signal power levelsfrom a node disposed down-stream of the low pass filtering circuitry;and